This invention relates to the manufacture of color television picture tubes incorporating shadow masks, and more particularly to such tubes utilizing the so-called "Black Matrix".
One serious drawback of the standard shadow mask tube is the appearance of its picture viewed in daylight conditions or high ambient lighting conditions. In the standard shadow mask tube there are two principal factors contributing to this deficiency. Firstly, in order to accommodate beam landing errors the diameters of the phosphor dots are made larger than those of the beams, and secondly the spaces surrounding these dots is covered by a highly reflective aluminum coating. These two factors together mean that approximately 10% of the screen area is never excited by any electron beam but will nevertheless diffusely reflect ambient light and hence impair the appearance of the picture. In order to reduce this effect to acceptable limits a dark-tint face plate is used. This reduces, by a certain factor, the brightness of the reflected ambient light; but it also reduces, though only by half that factor, the brightness of the picture. Thus although there is a net improvement in appearance, the general level of picture brightness is reduced. This effect can be compensated by driving the phosphor harder, but this reduces the life of the tube. Taking this fact into consideration a face plate with approximately 50% transmission is generally considered a reasonable compromise.
A novel approach to the problem is firstly to arrange to accommodate beam landing errors by making the electron beams larger than the visible portions of the dots, and secondly to fill the space between these portions with a non-reflecting coating.
With this arrangement, if only 50% of the screen area is excited by electron beams and the remainder is substantially non-reflecting, then an 80% transmission face plate could be used, thereby providing a 54% increase in brightness with a 30% better contrast ratio between the picture and the reflected ambient light.
The arrangement can be achieved in principle by producing an appropriate pattern of holes in a black coating on the inside of the tube facce. The holes are made undersize on the standard phosphor dots which are subsequently superimposed on these holes. The diameters of the electron beams are again standard size but beam landing errors are accommodated by virtue of the fact that the effective light emitting area of each phosphor dot is restricted to that portion lying in its associated hole.
A critical step in this process is the production of the pattern of undersize holes. Since a shadow mask is unique to a tube this pattern can only be achieved by a technique which involves the temporary reduction in size of the apertures of a shadow mask.
Two methods have previously been proposed. One of these involves making the shadow mask with undersize holes which are subsequently enlarged by etching after it has been used to make the pattern of undersize holes in the black coating. The other method involves using a shadow mask with standard size holes which is then plated with a different metal to reduce their size. After the pattern of undersized holes in the black coating has been made the holes of the shadow mask are opened out again to their original size using a selective etch which will remove the plated layer, but which will not attack the underlying material from which the shadow mask is constructed. Both these methods are complicated by the fact that the enlargement of the holes has to be performed after the shadow mask has been fitted to its supporting frame and formed. It will be realized that there is a risk of trapping the etching solution between the mask and its frame which may cause contamination within the completed tube. A further disadvantage of the plating method is that it is time consuming and expensive on materials. Yet another disadvantage of methods involving the use of etching solutions is that they are liable to remove the oxide layer created during the forming of the shadow mask. If this oxide layer has to be recreated by a further heat treatment there is a serious risk that the shadow mask will slightly change its shape and thus lose its compatibility with the deposited pattern of phosphor dots.